ph concept

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rachel liu 3k
Posts: 56
Joined: Sat Aug 17, 2019 12:16 am

ph concept

Postby rachel liu 3k » Sat Dec 07, 2019 7:52 pm

please quote my question so i get a notification, thanks in advance!

i know there is something wrong with my thought process, but i cannot figure it out. my thought process is that if a base is weaker, less oh- will be dissociated out. taking the -log of that smaller number will result in a smaller pOH, and therefore a higher pH.

i know i'm supposed to be getting a less basic solution, or a lower pH and a higher pOH, but i cannot seem to figure out what is wrong with this statement. hope you can help, thanks!

Sebastian Lee 1L
Posts: 157
Joined: Fri Aug 09, 2019 12:15 am
Been upvoted: 1 time

Re: ph concept

Postby Sebastian Lee 1L » Sat Dec 07, 2019 7:59 pm

rachel liu 3k wrote:please quote my question so i get a notification, thanks in advance!

i know there is something wrong with my thought process, but i cannot figure it out. my thought process is that if a base is weaker, less oh- will be dissociated out. taking the -log of that smaller number will result in a smaller pOH, and therefore a higher pH.

i know i'm supposed to be getting a less basic solution, or a lower pH and a higher pOH, but i cannot seem to figure out what is wrong with this statement. hope you can help, thanks!


You're right in that less OH- will be dissociated in a weaker base. However, if you take the negative log of the small number you will actually get a larger value for pOH. Log functions make small values big and large values small. For example, if you had a concentration of OH- that was 10^-2 mol/L, taking the negative log gives you 2. If you had an even smaller [OH-] of 10^-5 mol/L, then you would get an even larger pOH: 5. Therefore, a weaker base with a smaller OH- concentration gives you a larger pOH value and a smaller pH. Good luck on the final! :)

vpena_1I
Posts: 109
Joined: Sat Aug 24, 2019 12:15 am

Re: ph concept

Postby vpena_1I » Sat Dec 07, 2019 7:59 pm

taking the -log of a smaller [OH-] gives you a higher pOH, making the pH smaller.

Eugene Chung 3F
Posts: 142
Joined: Wed Nov 15, 2017 3:03 am

Re: ph concept

Postby Eugene Chung 3F » Sat Dec 07, 2019 8:04 pm

rachel liu 3k wrote:please quote my question so i get a notification, thanks in advance!

i know there is something wrong with my thought process, but i cannot figure it out. my thought process is that if a base is weaker, less oh- will be dissociated out. taking the -log of that smaller number will result in a smaller pOH, and therefore a higher pH.

i know i'm supposed to be getting a less basic solution, or a lower pH and a higher pOH, but i cannot seem to figure out what is wrong with this statement. hope you can help, thanks!


If a base is weaker, less oh- will be generated. Since pOH is -log[OH-], smaller, low [OH-] will generate higher number. (ex: if [OH-]= 0.1, -log of that is 1. if [OH-] is 0.01, -log of that is 2, so lower [OH-]= higher pOH). Since pH = 14-pOH, the higher the pOH, the lower the pH. This makes sense since the weaker the base, the lower the pH will be.

rachel liu 3k
Posts: 56
Joined: Sat Aug 17, 2019 12:16 am

Re: ph concept

Postby rachel liu 3k » Sat Dec 07, 2019 11:08 pm

Eugene Chung 3F wrote:
rachel liu 3k wrote:please quote my question so i get a notification, thanks in advance!

i know there is something wrong with my thought process, but i cannot figure it out. my thought process is that if a base is weaker, less oh- will be dissociated out. taking the -log of that smaller number will result in a smaller pOH, and therefore a higher pH.

i know i'm supposed to be getting a less basic solution, or a lower pH and a higher pOH, but i cannot seem to figure out what is wrong with this statement. hope you can help, thanks!


If a base is weaker, less oh- will be generated. Since pOH is -log[OH-], smaller, low [OH-] will generate higher number. (ex: if [OH-]= 0.1, -log of that is 1. if [OH-] is 0.01, -log of that is 2, so lower [OH-]= higher pOH). Since pH = 14-pOH, the higher the pOH, the lower the pH. This makes sense since the weaker the base, the lower the pH will be.


This was super helpful! I think I was doing something wrong with my calculator because I thought the trend was higher OH concentration = higher pOH.

ramiro_romero
Posts: 90
Joined: Sat Sep 07, 2019 12:16 am

Re: ph concept

Postby ramiro_romero » Sat Dec 07, 2019 11:13 pm

rachel liu 3k wrote:please quote my question so i get a notification, thanks in advance!

i know there is something wrong with my thought process, but i cannot figure it out. my thought process is that if a base is weaker, less oh- will be dissociated out. taking the -log of that smaller number will result in a smaller pOH, and therefore a higher pH.

i know i'm supposed to be getting a less basic solution, or a lower pH and a higher pOH, but i cannot seem to figure out what is wrong with this statement. hope you can help, thanks!


A weaker base will not fully dissociate in solution, meaning a higher pOH and a lower pH. when pH is less than 7, (relatively low pH) then you know it is a base.


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